In Long Term Evolution (LTE), a scheme is specified in which a mobile station (UE) carries out LTE voice calls (VoLTE) by establishing an IP-CAN Bearer (SIP bearer), which is a logical communication path for Session Initiation Protocol (SIP) and voice media, on an IP-Connectivity Access Network (IP-CAN). IP-CAN is an access network providing a function to transport SIP and voice media (see, for example, Non-patent document 1).
FIG. 1 shows a communication sequence in a case where, after the mobile station establishes an IP-CAN Bearer, the IP-CAN Bearer is removed due to handover to UTRAN/GERAN. As shown in FIG. 1, when the mobile station executes handover to UTRAN/GERAN (S10), the IP-CAN Bearer is removed (removal) (S20).